This invention pertains generally to local area networks, and more particularly to methods and apparatus for implementing a distributed wireless local area network.
The flow of a wide variety of electronic information within the boundaries of a home or office has become a reality in today""s society. What began perhaps with a simple voice telephone line connection to the outside world has expanded to include cable television, digital television, telephone modems, satellite links, cable modems, ISDN (Integrated Services Digital Network) connections, DSL (Digital Subscriber Line) connections, local area networks, sophisticated security systems, intercom systems, multi-speaker xe2x80x9csurround soundxe2x80x9d entertainment, smart appliances and smart xe2x80x9chousesxe2x80x9d, etc. New technology will almost certainly expand the future uses for information distribution within the confines of a house or office.
With enough foresight, a new home or office can be equipped with what may be literally miles of wiring, to allow flexible configuration of a home or office to receive and distribute several (or perhaps all) of these forms of information. But once the walls are in place, adding wiring for a new technology, repairing wiring already in place, or even moving existing equipment to a new desired equipment location with no xe2x80x9coutletxe2x80x9d, may be reduced to choosing between either expensive remodeling or unsightly wiring running along baseboards and window sills. Furthermore, because most of these technologies require their own particular wiring and signaling requirements, a variety of wall sockets and wiring are required, all adding to the expense of construction and detracting from the aesthetics of the space.
Other problems with wired networks exist. For example, merging of multiple differing networks for centralized control, etc., requires expensive bridging, or bridges may not be available at all.
To combat these problems, wireless networks are now being designed for home use. Many of these networks work in the Industrial, Scientific, and Medical (ISM) band that exists at 2.400-2.4835 GHz. A second possible ISM band exists at 5.725-5.850 GHz. These bands allow unlicensed operation, as they are xe2x80x9cgarbagexe2x80x9d bands that are generally unsuitable for commercial broadcast use (microwave ovens, for example, operate in the 2.4 GHz band). Although low-power, narrowband signals may be jammed by the noise occurring in these bands, digital spread spectrum techniques can be used to effect useful bandwidth.
The Federal Communication Commission has recently created an Unlicensed National Information Infrastructure (U-NII) to further address the needs for wireless digital data communications, particularly for wireless transmission at a rate that can support multimedia. U-NII released three 100 MHz bands for use: 5.15-5.25 GHz, for indoor use only and at low power, suitable for short ranges such as within a room; 5.25-5.35 GHz, at an intermediate power for mid-range uses; and 5.725-5.825 GHz (overlapping the 5.7 GHz ISM band), at a higher power for use up to several miles. U-NII power requirements are designed to encourage wideband uses over narrowband uses, by specifying an allowable transmit power formula that reduces maximum output power logarithmically as signal bandwidth is reduced.
Within the ISM and U-NII bandwidth constraints, several network concepts have been designed, most notably the IEEE 802.11 format, the Bluetooth(trademark) format, and the Shared Wireless Access Protocol (SWAP) developed by the HomeRF Working Group. Each of these formats is designed for use in the 2.4 GHz ISM band. IEEE 802.11 format allows for data rates of 1 million bits per second (Mbps), 2 Mbps, and 11 Mbps, uses either Frequency Hopped Spread Spectrum (FHSS) or Direct Sequence Spread Spectrum (DSSS) to overcome noise, and has an operational range of about 40 m. SWAP allows for data rates of 1 or 2 Mbps, uses FHSS, and has an operational range of about 50 m. Bluetooth(trademark) format allows for a 1 Mbps data rate, uses FHSS, and allows for several operational ranges, depending on the power xe2x80x9cclassxe2x80x9d of the transceiver; the main applications for Bluetooth(trademark), however, envision the lowest power class transceiver, which has about a 10 m range.
In the IEEE 802.11 format, an xe2x80x9cad-hocxe2x80x9d network structure is envisioned. Each transceiver uses Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA), i.e., it listens for quiet on the channel before it transmits. FIG. 1 illustrates an CSMA/CA xe2x80x9cad-hocxe2x80x9d network formed with transceivers 20, 22, 24, and 26. Each transceiver can communicate with each other transceiver that is within its range, whenever the channel is not already in use. Problems can arise when two transceivers that are out of each other""s range (e.g., 20 and 26 in FIG. 1) cannot detect each other""s transmissions, and attempt to communicate simultaneously using the channel. This system also functions poorly with time-critical information, such as multimedia or voice.
SWAP is similar to IEEE 802.11 in many respects. But SWAP provides two access models, a Time Division Multiple Access (TDMA) service for time-critical data, and a CSMA/CA service for asynchronous data delivery. SWAP can work as an ad-hoc network as shown in FIG. 1. When time-critical services are in use, however, a Connection Point is required. The Connection Point coordinates the TDMA service such that sufficient bandwidth is reserved for the time-critical services. This system""s TDMA mode overcomes some of the problems of IEEE 802.11, although bandwidth is more limited.
FIG. 2 illustrates the more structured wireless concept employed by Bluetooth(trademark), as described in the Bluetooth Specification Version 1.0B, Nov. 29, 1999. The Bluetooth(trademark) unit of network service is termed apiconet, e.g., 46, 48, 50, each of which comprises one master transceiver (28, 34, 40, respectively) and up to seven slave transceivers. Within each piconet, a FHSS channel and phase is established by the master, unique to that master. TDMA is used with 625 microsecond timeslots, with the master communicating in even-numbered time slots. In odd-numbered time-slots, the slave last addressed by the master is allowed to communicate. Each time-slot, the frequency for the piconet is hopped to the next in the hopping sequence established by the master. Slave transceivers follow the hop sequence for that piconet, communicating with the master when allowed by the master.
A scatternet 52 is a group of piconets with overlapping coverage areas. Because each piconet operates on a different FHSS channel, frequency conflicts are infrequent. When conflicts do occur, each piconet may lose a single packet. Although a single transceiver is allowed to be a master in one piconet and a slave in another (e.g., transceiver 34), or a slave in two piconets (e.g., transceiver 38), effective dual-piconet operation can be difficult to establish and maintain, since the specification establishes that overlapping piconets shall not be time- or frequency-synchronized. Furthermore, although a transceiver may have visibility in two piconets, this does not establish visibility between other transceivers in overlapped piconets. Each connection in each piconet allows only for communication between that piconet""s master and one of its slaves.
This structured design has advantages and disadvantages over the other formats described. It provides rigid control that is useful for time-critical applications and xe2x80x9cplug and playxe2x80x9d operation, and allows for devices to exist in multiple piconets. Lower power requirements decrease interference between overlapping piconets, allowing each piconet to enjoy most of its potential 1 Mbps throughput. But range is limited to less than typical household dimensions, bandwidth is inadequate for multimedia, the structure forces communication only with the master (slaves cannot communicate with each other during their time slots), the number of active devices in a piconet is severely limited, and the structure can waste bandwidth because the master must use an entire time slot each time it gives permission for transmission.